[client] egl: add new downscale filter

client/renderers/EGL/shader/downscale.frag

@@ -0,0 +1,26 @@
+#version 300 es
+precision mediump float;
+
+in  vec2  fragCoord;
+out vec4  fragColor;
+
+uniform sampler2D texture;
+uniform vec3      uConfig;
+
+void main()
+{
+  float pixelSize = uConfig.x;
+  float vOffset   = uConfig.y;
+  float hOffset   = uConfig.z;
+
+  vec2 inRes  = vec2(textureSize(texture, 0));
+  ivec2 point = ivec2(
+    (floor((fragCoord * inRes) / pixelSize) * pixelSize) +
+    pixelSize / 2.0f
+  );
+
+  point.x += int(pixelSize * hOffset);
+  point.y += int(pixelSize * vOffset);
+
+  fragColor = texelFetch(texture, point, 0);
+}